In deep sub micron microelectronic manufacturing, silicon nitride films are widely employed as stop layers during contact etching. As is well known in the art, the inter-level dielectric (ILD) is formed to provide insulation between the MOS devices and overlying metal lines that will be formed subsequently. The contact openings are made through the ILD layer to the source, drain and gate. The ILD layer is relatively thick, and it is hard to control the etching process in order to not over etch. The contact etch stop layer is formed prior to the formation of the ILD layer. A first highly selective etching is conducted to etch the ILD layer and stop at the etch stop layer. A second selective etching is conducted to etch the stop layer so that the underlying layer is exposed.
The conventionally used silicon nitride etch stop layer has several disadvantages. It has a high dielectric constant value (also known as k value), typically about 7.5 to 8. The high k value increases high parasitic capacitance, which degrades the device performance. Additionally, the deposition rate of the silicon nitride film is low and the deposition temperature is high, which are not desirable process characteristics.